This invention relates to radio determination by artificial earth satellites, and more particularly to a method of navigation or position surveillance using satellites transmitting timing signals with a correction by a two-way active ranging measurement. This method can provide a position fixing accuracy within a tenth nautical mile and requires user clock accuracy comparable to that of a crystal clock.
A need is recognized for continuous readout of position aboard a ship to enable the ship to steer for minimum fuel or minimum time. An object of this invention is to provide a system for ships that have a satellite communication system to also have the desired navigation service continuously and at low cost. Several presently known satellite systems do not have the capability for this navigation service on a worldwide basis or have some disadvantage. The planned Global Position Fixing System requires relatively expensive user equipment. The TRANSIT system could provide the service intermittently by providing accurate fix readouts and then using a dead reckoning computer between fixes, but the long time between fixes causes the dead reckoning computer to accumulate unacceptable errors. The Omega system will be worldwide and continuously available but does not have the desired 0.1 nautical mile accuracy. LORAN C can supply the service in limited areas, but worldwide coverage by LORAN C is not foreseen.
One-way ranging systems utilizing very accurate cesium beam clocks are described in the inventor's U.S. Pat. No. 3,384,891, which was granted on May 21, 1968 and is assigned to the assignee of this invention. Two geostationary satellites in view of an ocean basin each transmit a digital timing code continuously containing a timing signal such as a marker each second and information on the satellite location. A user of the signal who does not know his location but who has a clock accurately set to the time standard being transmitted by the satellite can measure the time of arrival of the signal relative to his clock and thus determine range from the satellite to himself. Radio signals travel at the speed of light, and the user can determine the travel time from the satellite to his location and thus a correction for propagation delay on the received time code signal. Knowing the exact position of the satellite, he can then determine a line of position. The use of two satellites enables him to determine his position fix at the intersection of the two lines of position from each of the two satellites. A very accurate clock is essential in such one-way ranging systems because an error of one microsecond in its time relative to the system reference standard will introduce an error of 1000 feet in the range measurement from the satellites. The cost of cesium beam clocks, which have an accuracy of 1 part in 10.sup.11 or better, would be excessive for this application.
The present invention makes possible a continuous navigation or position surveillance service with user crystal clocks and satellites that relay standard time signals, with an occasional active range measurement through an active ranging and communications satellite or timing signal satellite. Position surveillance by another method is disclosed and claimed in concurrently filed application Ser. No. 842,401 and in continuation-in-part application Ser. No. 899,117 filed on Apr. 24, 1978 by the inventor, which is assigned to the same assignee as this invention.